Ink jet printing systems produce images by printing patterns of data on a sheet of print media, such as paper. Such systems typically include two main mechanisms for determining the location of dots on the page, namely, a halftone mechanism and a shingling mechanism. Such mechanisms may be implemented, for example, in software, firmware, hardware, or a combination thereof, and may reference one or more lookup tables.
The halftone mechanism compensates for the inability of an ink jet printer, i.e., a binary printer, to print a continuous range of tones. For example, a binary printer can only produce colors by either printing or not printing a dot of ink, which at first glance would suggest only two shades can be printed. However, by printing patterns of various percentages of ink in a given area between zero percent and 100 percent ink, in effect many shades can be produced. Thus, halftoning involves the eventual pattern of dots placed on a page as a result of the printing process and the halftone pattern is visible to the user after the page is completely rendered.
Once the halftone mechanism has decided where the dots of ink are to be placed on the page, the shingling mechanism decides on which pass of plurality of passes of the ink jet printhead over a given prim area that particular dots of ink are to be deposited. Whereas it may be possible to print all of the dots on a single pass over a given area, in general multiple passes are used to hide horizontal bands. It is the function of the shingling mechanism to deposit all of the dots once, and only once, in the positions determined by the halftone algorithm.
In contrast to halftoning, patterns relating to shingling ideally come into play only while the page is printing and are theoretically of no concern once the job is finished as all of the dots have been placed according to the halftone pattern once the job is done. Traditionally there has been more interest in patterns relating to halftoning than in patterns relating to shingling since customers are concerned about the quality of the document after it has been completely printed rather than intermediate states that only exist momentarily as the document is printed.
According to conventional 2-pass shingling, wherein the variable N represents the number of printing passes, on a single pass of the printhead 1/N=½=50% of the dots are printed, commonly using a “checkerboard” pattern, which is defined by a shingling mask. The shingling mask determines for given area the pixel locations within that area that may receive a dot of ink in a particular printing pass, and the pixel locations that will not receive a dot of ink on that particular printing pass. Between passes, the paper is moved with respect to the printhead by a distance equal to 1/N=½=50% of the height of the printhead. Additionally, between passes of the printhead the shingling pattern is changed from one phase of the checkerboard shingling mask to the complementary phase of the checkerboard shingling mask. By advancing the paper and changing the shingle mask phases, all of the dots at each of the pixel positions in the checkerboard pattern have one and only one chance to be printed.
While shingling at N=2 was used in the example described above, it is also known to perform shingling where the number of passes is greater than 2, e.g., where N=3, 4, 6, 8 and 16. The ideas are the same as for N=2 pass shingling in that a fraction of approximately 1/N of dots requested by the halftone to eventually be deposited is placed on each pass, and the paper is advanced by about 1/N of the fractional height of the printhead. Additionally, the shingling pattern of each level of a shingle mask is extended beyond that of a checkerboard to a different, but typically small, repeating pattern.
Various halftoning algorithms have been implemented that attempt to reduce the generation of undesirable printing artifacts, i.e., the printing of unwanted information, resulting form repetitious halftoning patterns used in attempting to print a desired image. One such halftoning algorithm is known in the art as error diffusion halftoning.
In addition, print artifacts may be present in the multiple shingling mask phases of printing the halftone in multiple passes. For example, print artifacts may be present in the two phases of the exemplary 2-pass checkerboard pattern when the ink dots are not placed mutually exclusively of one another in the checkerboard patter. Instead, the dots overlap with adjacent neighbors between the two passes. The result, for example, is an ordered pattern of white paper showing through what should otherwise be a solid black square.
Several attempts have been made to overcome the problem of the objectionable periodic defects caused by the ordered nature of the shingling pattern through the use of random or pseudo-random patterns of dots on each pass of the printhead instead of small repeating patterns, wherein random number are used to generate the shingling masks. However, the use of random shingling masks also produces artifacts which are related to the arrangements of the dots in the random shingling patterns. For example, the random patterns may posses both high and low frequency image content as is characteristic of a random signal. The presence of the low frequency information is undesired, since the human visual system is known to be sensitive to low frequencies. Such low frequency information present in the random shingling masks may be observed in the printed result as an objectionable grain pattern.